The present invention relates to an insoluble electrode used for electrolytic treatment of an aqueous solution and a process for the preparation of such electrode. More particularly, the present invention relates to a process for the preparation of insoluble electrodes having few surface defects, which comprises coating the surface of an electroconductive, corrosion resisting base metal, such as titanium, niobium, zirconium, tantalum, an alloy thereof, or other electroconductive, corrosion resisting base metal, with at least one layer of the platinum group metals and irradiating the coated surface with laser beams in an oxidizing or non-oxidizing atmosphere. Furthermore, the present invention relates to long-life insoluble electrodes prepared by such a process.
Insoluble electrodes are frequently used as electrodes in the electrolytic industry. As the typical processes for the preparation of these insoluble electrodes, there have been adopted (1) a process comprising plating a metal of the platinum group on an electroconductive, corrosion resisting base metal, such as titanium, and (2) a process comprising plating a metal of the platinum group on such an electroconductive base metal and then subjecting the plated base metal to a heat treatment.
Electrodes prepared according to these conventional processes, however, are inevitably defective in various points and are not practically suitable for industrial-scale applications.
The conventional electroplating process will now be described with reference to FIG. 1.
FIGS. 1A through 1D are diagrams illustrating the relation of the deposition state to the deposition amount and plating thickness, which is observed when platinum is plated on an electroconductive base metal consisting of titanium. When the deposition amount of platinum is small such as 0.2 .mu.m, as shown in FIG. 1A, the absolute amount of plated platinum is small and the platinum is deposited only locally, so that the surface of the resulting electrode contains many defects. Even if the deposition amount of platinum is increased to 1 or 3 .mu.m, the platinum tends not to become deposited on new areas of the electroconductive base metal consisting of titanium but rather preferentially grows on the already deposited platinum; thus, the platinum does not completely cover the titanium surface. It is only when the deposition amount of platinum becomes large, such as about 7 .mu.m, that the titanium surface is substantially covered. However, such an increase of the plating thickness increases the plating cost. Also, such a large amount of platinum often causes coarse crystals to be formed. In such a case, the resulting electrode is defective in that a coating with many pinholes is liable to form and the adherence of the plating layer to the electroconductive base metal is poor. There is a preparation process, in which striking plating may be carried out beforehand, but the complete prevention of defects cannot be attained.
If defects, such as pinholes, are present on the surface of an electrode plated with the platinum group metals, the current concentrates around the pinholes, especially when electrolysis is carried out at a high current density, and cracks form around the pinholes, resulting in peeling of the plating layer and extreme shortening of the life of the electrode.
More important, as disclosed in Japanese patent application Laid-Open No. 53-87938 filed by the assignee here of, and laid open Aug. 2, 1978, during electrolytic treatment, the stoppage of application of the electric current cannot be avoided, and when the electric current is not applied, the potentials of the anode and the cathode are reversed; that is, the anode becomes cathodic and the cathode becomes anodic. Accordingly, since reversion of the potentials is repeated when the application of the electric current is stopped and resumed, the life of the electrode is shortened, and if there are pinholes present when the electrode is used under the above conditions, corrosion of the electroconductive base metal by the repeated reversion of the potentials starts from the location of the pinholes, with the result that peeling of the platinum plating layer takes place and the life of the electrode is further shortened.
As a means for eliminating the surface defects caused by electroplating, there has been adopted a method in which the heat treatment is carried out in a heating furnace or flame. It is said that pinholes are removed and the plating layer is alloyed with the electroconductive base metal to improve the adhesion and corrosion resistance. However, even if this heat treatment is conducted, it is difficult to obtain an electrode having the desired characteristics.
More specifically, a temperature higher than 600.degree. C. is necessary so as to induce diffusion between the electroconductive base metal and the platinum group metals plated on the electroconductive base metal. Due to a conventional heat treatment at a temperature higher than 600.degree. C., the electroconductive base metal is deformed, and the diffusion between the electroconductive base metal and the platinum group metals plated on the electroconductive base metal becomes difficult to control, grain coarsening of the electroconductive base metal and platinum group metals takes place, and cracks are formed. Furthermore, since the conventional heat treatment at a high temperature must be carried out over a long period of time, the mechanical strength and electric conductivity of the electroconductive base metal become deteriorated, due to oxidation in the case of the heat treatment in an oxidizing atmosphere and due to the formation of nitrides in the case of the heat treatment in a nitrogen atmosphere. Therefore, the heat treatment has usually been carried out in a vacuum.
Referring to FIG. 2, there is illustrated an example of the microscopic structure of a cross section of a platinum-plated titanium electrode which has been heat treated in a vacuum by a conventional process. More specifically, the heat treatment was carried out at 1000.degree. C. over a period of 15 minutes in a vacuum. A thick and coarse alloy layer comprised of Pt.sub.3 Ti and PtTi.sub.3 was grown by the heat treatment, as seen in FIG. 2. The electrode, having the microscopic structure as shown in FIG. 2, has a short life because of the reasons which will be explained in detail later. Selection of appropriate conditions for the formation of an alloy layer and appropriate conditions for preventing oxidation or nitriding of the electroconductive base metal are very difficult and it also is difficult to control the diffusion of the plated metal in the conventional heat treatment as explained hereinabove.
It was previously proposed, in assignee's Japanese Laid Open patent application No. 56-47597 laid open Apr. 30, 1981, a process for the preparation of the platinum-plated titanium electrodes in which the foregoing disadvantages are eliminated. According to this process, in order to prevent the formation of pinholes during electroplating of the platinum group metals and to remove the undesired influences of the heat treatment conducted at a high temperature, a solution of a compound of the platinum group metals is coated on a plating layer of the platinum group metals after electroplating and the coated base metal is heated at a relatively low temperature in a non-oxidizing atmosphere to effect thermal decomposition and thermal diffusion.
The defects and disadvantages involved in the conventional techniques can considerably be eliminated, according to this process, but since a C1, NO or NO.sub.2 compound is used as the platinum group metal compound to be coated and since decomposition is carried out at a relatively low temperature, the decomposition is insufficient and there is a risk that impurities, such as Cl, NO and NO.sub.2, will be left in the plating layer, thereby reducing corrosion resistance. Furthermore, since the heat treatment is conducted at a low temperature, the adhesion of the plating layer is not sufficient.
Japanese Laid Open patent application No. 52-20988 laid open Feb. 17, 1977 (corresponds British Provisional Specification No. 47235/1974); and No. 56-119787 laid open Sept. 19, 1981 (corresponds U.K. patent application GB No. 2067537A) describe the processes of making insoluble electrodes by means of laser beam irradiation. In the former Japanese Laid Open patent application, it is disclosed that laser beams are directly applied onto the surface of an electroconductive base metal, so as to improve its qualities, while in the latter Japanese Laid Open patent application the surface of an electroconductive base metal is directly coated with a metal oxide and then laser beams are applied onto the coated surface. In the process in which the laser beams are directly applied onto the electroconductive base metal, the quality improvement due to the laser beam irradiation is appreciable, but a good corrosion resistance cannot be achieved, because the inherent corrosion resistance of the base metal is not sufficient for that required for insoluble electrodes. On the other hand, in the process in which the direct coating of a metal oxide on an electroconductive base metal is followed by laser beam irradiation, it is difficult to form a continuous layer by means of applying the metal oxide. In addition, as described in detail hereinbelow, a diffusion layer of metal oxide and the electroconductive base metal is hardly formed so that the coating formed is not sufficient for the protection of such base metal. This is one of the problems involved in the process mentioned above.